Human placental ferredoxin is a [2Fe-2S] vertebrate ferredoxin with 124 amino acid residues. It functions to transfer electrons from NADPH-dependent ferredoxin reductase to cytochrome P450 enzymes. This protein has been overexpressed directly in E. coli by using the T7 promoter/T7 polymerase expression system (yield: /30 mg/L culture). Here, we report the assignments of the histidine imidazole side-chain NMR signals resonances of human ferredoxin. The histidines are of interest because one histidine residue (His56), which is conserved in all vertebrate ferredoxins, is directly adjacent to one of the iron-sulfur cluster ligand cysteines--Cys55. Signal overlap and spectral complexity in the proton NMR spectrum led to ambiguities in previous studies of the histidines of bovine adrenodoxin. We overcame this problem by using multinuclear NMR spectroscopy. By labeling human ferredoxin uniformly with 15N and selectively by incorporating [13C] histidine, we were able to collect two-dimensional 1H{13C, 15N}MBC and 1H{13C}SMQC data. Almost all of the 1H, 13C and 15N imidazole resonances of the three histidines have been assigned. In addition, the backbone assignments from 3D NMR experiments data of 15N labeled human ferredoxin will be performed. (Supported by USDA Grant 92-37306-7699 and NSF Grant MCB-9215142; NMR spectroscopy was carried out at NMRFAM, which has partial support from NIH Grant RR02301).